Hybrid thermoelectric generator

ABSTRACT

A CYLINDRICAL THERMOELECTRIC GENERATOR HAVING TWO CONCENTRIC THERMOELECTRIC ELEMENT IS DISCLOSED. THE OUTER ELEMENT IS USED TOGETHER WITH THE HEAT RECEIVING AND HEAT REJECTION MEANS AND APPROPRIATE BONDING AND STRESS ALLEVIATION MEMBERS, OT HERMETICALLY SEAL UNDER COMPRESSION THE INNER ELEMENT FROM THE ENVIRONMENT SURROUNDING THE GENERATOR.

H531 4, i972 L. J. CAPRAROLA 3,6745@ HYBRID THERMOELECTRIC GENERATOR Filed April 1, 1968 ATTQRNEY Patented July 4, 1972 3,674,568 HYBRID THERMOELECTRIC GENERATOR Leroy J. Caprarola, Morristown, NJ., assiguor t RCA Corporation Filed Apr. 1, 1968, Ser. No. 717,526 Int. Cl. H01v 1/02 U.S. Cl. 13G-205 4 Claims ABSTRACT OF THE DISCLOSURE A cylindrical thermoelectric generator having two concentric thermoelectric elements is disclosed. The outer element is used together with the heat receiving and heat rejecting means and appropriate bonding and stress alleviation members, to hermetically seal under compression the inner element from the environment surrounding the generator.

BACKGROUND 0F INVENTION This invention relates to thermoelectric couples and more particularly to a thermoelectric couple having two active semiconductor thermoelements, one being P-type and the other being N-type, where one of the thermoelements is used as part of the means for hermetically sealing and protecting the other from the surrounding environment.

When two dissimilar thermoelectric elements (hereinafter called thermoelements) are thermally joined to form a loop, and the two junctions forming the loop are kept at different temperatures, a certain electrical power is produced due to what is known as the Seebeck eifect. The eiciency of this device (hereinafter called a thermocouple) is directly proportional to the difference in temperature between the hot junction and the cold junction. When used herein, hot junction will mean the junction to which heat is applied and cold junction will mean the junction from which heat is transferred into the environment surrounding the thermocouple.

It has further been found that where the thermoelements are semi-conductive materials, appropriately doped with either P-type or N-type impurities, a higher etiieiency is obtained than in a system where purely metallic elements are used. The eiciency is given approximately by the formula,

MlAMZ) (AT) where Z is the gure of merit of the material used and AT is the temperature difference between the hot and cold junctions. Every type of material has a diterent iigure of merit and, in fact, a P-type and an N-type of the same host material may have widely differing figures of merit.

It has further been found that by using one thermoelement of a P-type semiconductor and another thermoelement of an N-type semiconductor, a greater power may be obtained. From this it may appear that all that is necessary for a good thermoelectric generator is to nd the P-type semiconductor with the highest tigure of merit and the N-type semiconductor with the highest gure of merit, and combine the two into a thermocouple. However, certain important physical properties of thermoelectric materials which must |be ta-ken into consideration, especially at high temperatures, make the problem far more complex.

At high temperatures, certain materials tend to sublime, become ineiiicient, or not ofer long-term stability. A further problem is that certain materials are diticult to metallurgically bond to other mem-bers in the device. Another problem is that certain materials do not have high tensile strength, and hence the thermocouple cannot be used for many desirable purposes.

Various solutions to these problems individually exist although no solution, other than the one taught herein, can utilize the advantages of certain material and construction techniques to overcome the disadvantages of others.

It has been found that the problem of low tensile strength can be overcome by placing a thermoelement of the low tensile strength material under a certain built-in compression. When the thermoelement is under compression it is further unnecessary to use strong metallurgical bonds between the thermoelement and other members of the thermocouple.

The tendency of certain materials to breakdown at high temperatures has been eliminated by segmenting the material at the hot end with another material of the same polarity which doesnt breakdown at the high operating temperature. This technique may also be used for those materials which exhibit a high tigure of merit at low temperatures but a low one at high temperatures.

The problem of sublimation is corrected by hermetically sealing the material in an environment of an inert gas. However, in the past, the means used for hermetically sealing the thermoelement utilized a non-semiconductive material, such as stainless steel, which further acted as the second thermoelement. This added weight and complexity to the system without an equivalent amount of added efficiency. `It further shunted heat from the semiconductive thermoelement, and thus reduced power output and eiciency.

Through the use of this invention, it is possible to utilize the procedures for overcoming the problems of low tensile strength, difficult bonding, breakdown, sublimation and iigure of merit selection without having to hermetically seal the material in a non-semiconductor material.

Therefore an object of this invention is to provide an improved thermocouple.

Another object is to provide a lighter and more powerful and efficient thermocouple.

Yet another object is to provide a thermocouple in which the problems of low tensile strength and difficult bonding of materials is overcome.

These objects are realized by using a doped semiconductive material as one thermoelement, where the material has a high stability and figure of merit, is insensitive to its environment, is easily bondable, and possesses high tensile strength, and by further using this same thermoelement as part of the apparatus for hermetically sealing a second thermoelement under compression where the material of the second thermoelement does not possess some of the above-mentioned desirable characteristics.

The objects and novel features of the invention will become apparent when taken in connection with the two figures, the detailed description and appended claims wherein:

FIG. l shows a cut away View taken across the diameter of a first embodiment of a cylindrical thermocouple having two concentric cylindrical thermoelements, and

FIG. 2 shows a second embodiment of the same type cf thermocouple.

In FIG. 1, a cylindrical thermocouple is constructed with solid cylindrical inner thermoelement 10 and hollow cylindrical outer thermoelement 12. The inner wall of thermoelement 12 has a larger diameter than the outer wall of thermoelement 10. Therefore, when thermoelement 10 is placed concentrically inside thermoelement 12, gap 14 exists. Thermoelement 12, together with heat reception member 16, heat rejection member 18 and associated bonding and stress alleviation members, insulators and terminals, which will be hereinafter explained in more detail, will hermetically seal thermoelement under a compression from the environment in which the thermocouple operates.

The choice of the thermoelectric materials for thermoelements 10 and 12 is based on several considerations. The host material of thermoelement 12 must be capable of unprotected operation in a variety of environments, such as air, vacuum, or combustibles. It should possess high tensile strength and be easily bondable to other materials by metallurgical methods.

Thermoelement 10 need not possess any of the above features. Since it is hermetically sealed from the thermocouples environment, its operation is independent of the environment in which the thermocouple operates. Because thermoelement 10 is under compression, its tensile strength becomes a minor factor and it can be contacted to other materials due to this compression alone, if necessary, or Vit may be metallurgically bonded.

The materials of both thermoelements 10, 12 must possess high figures of merit and exhibit stable operation at high temperatures. As will be explained in more detail hereinafter, it is possible to segment the thermoelements 10 and 12 with other materials at the hot and cold ends to achieve a maximum figure of lmerit and stability of operation.

By way of example, P-type silicon-germanium can be used for thermoelement 12 and N-type lead telluride can be used for thermoelement 10. P-type silicon germanium Ehas high tensile strength, is easily bondable by metallurgical methods, is insensitive to operating environments, operates well at high temperature, and has a high figure of merit. N-type lead telluride has a high figure of merit.

At its hot junction, thermoelement 12 is coupled to heat reception plate 16 through bonding member 19 and stress alleviation member 20, each of which are thermal and electrical conductors. The material of member 19 may be tungsten and the material of member 20 may be gold. Thermoelement 10 at its hot junction is coupled to heat reception plate 16 through bonding member 22 and stress alleviation member 24, both of which are electrical and thermal conductors. Member 22 may be iron and member 24 may be gold. Heat reception plate 16 is also a thermal and electrical conductor such as tungsten. Hence an electrical path exists at the hot junctions between thermoelement 10 and thermoelement 12 for electrons, and thus current, to flow due to the Seebeck effect.

At the cold junction, thermoelement 12 is coupled to terminal 26 through bonding member 28. The material of bonding member 28 may be tungsten, and terminal 26 can be made of nickel plated copper. In order to insure continuity of contact and to obtain hermetic sealing, a moveable L-shaped circular brazing collar 29 of nickel plated copper snugly fits around bonding member 28 and is placed in intimate contact with the top of terminal 26. The cold junction of thermoelement 10 is coupled through bonding member 30 and stress alleviation member 32 to terminal 34. Member 30 may be iron and member 32 gold. Terminals 26 and 34 are electrically, but not thermally, separated by insulator 36 which may be A1203. Terminal 34 is thermally, but not electrically, coupled at heat rejection plate 18 through insulator 38, which may be A1203.

Since terminals 26 and 38 are not electrically coupled, a voltage exists between them. When these terminals are coupled together through a load, electrical power is derived from the thermocouple.

Heat may be supplied to heat reception plate 16 from the sun, by radiation from a radioisotope fuel capsule or from the mandril of a fossil fuel burner.

Stress alleviation members 24 and 32 and bonding members 22 and 30 and thermoelements 10 and 12 should -be of such dimensions that thermoelement 10 is put lmder a slight compression.

In applications where such is desired, an inert gas, for

example, percent argon and 5 percent hydrogen, can be backlled into gap area 14 between thermoelements 10 and 12. While not shown, a suitable, scalable tubulation can be provided through the heat rejection member 18, insulator 38, terminal 34 and stress alleviation member 32 to provide access to the gap area 14 for the insertion of the inert gas.

In FIG. 2, a second embodiment is shown for use where very large temperature differences are necessary such as l000 C. at the hot junction and 100 C. at the cold junction.

Heat reception member 40 is directly bonded to P-type silicon germanium thermoelement 42. Member 40 in this embodiment is a semiconductor material with good electric and thermal conducting properties such as silicon molybdenum, which is highly doped with P-type impurities. Member 40 may be directly coupled to inner thermoelement 44, as shown in FIG. 2, or may be coupled through stress alleviation and bonding members as previously shown in FIG. 1.

Since N-type lead telluride breaks down at high temperatures, it is necessary to segment thermoelement 44 with N-type silicon germanium on the hot side. Where very low temperatures are found at the cold junction, it is desirable to segment the N-type lead telluride of thermoelement 44 with N-type bismuth telluride. In FIG. 2, thermoelement 44 is shown as composed of a segment of N-type silicon germanium 46, a segment of N-type lead telluride 48 and a segment of N-type bismuth telluride 50. Between segment 46 and segment 48 is bonding member 52 composed of tungsten and between segment 48 and segment 50 is a bonding member 54 composed of iron. The lower portion of FIG. 2 is identical to that of FIG. 1. For convenience, however, the parts of FIG. 2 similar to those in FIG. 1 have been identified with the same numerical designations, and reference is made to the description previously given for specific details.

What is claimed is:

1. A thermoelectric couple comprising,

(a) -frst and second semicondctor thermoelements, one of which is doped with P-type impurities and one of which is doped with N-type impurities, said first thermoelement including a semiconductive material which is not sensitive to the environment in which the thermoelectric generator operates, which possesses high tensile strength, and which is easily bondable by metallurgical methods, and

(b) encasing means, including said first thermoelement, for isolating said second thermoelement from the environment in which said thermoelectric couple operates, said encasing means further comprising,

(1) coupling means, including a heat reception member for receiving heat from a source of heat, for electrically coupling said first thermoelement to said second thermoelement, and

(2) output means coupled between said thermoelements and deriving electrical power therefrom,

said output means further comprising a first bonding member coupled to said first thermoelement, a first electrical terminal member coupled to said first bonding member, a first electrical insulator coupled to said first electrical terminal member, a second bonding member coupled to said second thermoelement, a stress alleviation member coupled to said second bonding member, a second electrical terminal member coupled to said stress alleviation member `and said first electrical insulator, a second electrical insulator coupled to said second electrical terminal member and a heat rejection member coupled to said second electrical insulator, wherein said bonding members, stress alleviation member, and electrical members are thermal and electrical conductors and wherein said electrical insulators are thermal conductors and electrical insulators, the dimensions of said rst thermoelement, said second thermoelement, said coupling means and said output means placing said second thermoelement under compression.

2. A thermoelectric couple comprising,

(2) output means coupled between said thermoelements and deriving electrical power therefrom, said host material of said heat reception member comprising silicon molybdenum, the

(a) -iirst and second semiconductor thermoelements, one 5 dimensions of said first thermoelement, said secof which is doped with P-type impurities and one of ond thermoelement, said coupling means and which is doped with N-type impurities, said rst said output means placing said second thermothermoelement including a semiconductive material element under compression. which is not sensitive to the environment in which 4. A thermoelectric couple comprising, the thermoelectric generator operates, which possesses (a) first and second semiconductor thermoelements, high tensile strength, and which is easily bondable one of which is doped with P-type impurities and one by metallurgical methods, and of which is doped with N-type impurities, said rst (b) encasing means, including said first thermoelement thermoelement including a semiconductive material for isolating said second thermoelement from the which is not sensitive to the environment in which environment in which said thermoelectric couple opthe thermoelectric generator operates, which possesses erates, said encasing means further comprising, high tensile strength, and which is easily bondable (l) coupling means, including a heat reception by metallurgical methods, and

member for receiving heat from a source of heat, (b) encasing means, including said first thermoelement, for electrically coupling said first thermoelement for isolating said second thermoelement from the to said second thermoelement, and environment in which said thermoelectric couple op- (2) output means coupled between said thermoerates, Said eIICaSiIlg means further Comprising,

elements and deriving electrical power there- (1) ccilPliIlg means, including a heat reception from, member which is a highly electrical and thermal said coupling means further comprising a iirst bonding Conducting host Semiconductor material doped member coupled to said iirst thermoelement, a first With P-tYPe impurities for receiving heet from a stress alleviation member coupled between said irst SOlllce 0f heat and fOr electrically cOllPlIlg Said bonding member and said heat reception member, a ifSt thermoelement t0 Said SecOIld thelmOelesecond stress alleviation member coupled to said heat ment, and reception member, and a second bonding member (2) Output means coupled betWeen Said thermOelecoupled between said second stress 'alleviation mem- 30 ments and deriving e1ectficdl POWel therefrom, ber and said second thermoelement, wherein said heat Said heat recePtiOii member being bonded directreception member, said bonding members and stress ly t0 Seid iil'St thermoelement and coupled alleviation members are thermal and electrical contlliciigll a StfeSS relief member and e bonding ductors, the dimensions of said iirst thermoelement, member t0 Said Second thermoelement, the disaid second thermoelement, said coupling means land meiiSiOilS 0f Said i'ifSt thermoelement, Said Secsaid output means placing said second thermoelement 0nd thermoelement, Seid coupling means and under compression, said output means placing said second thermoele- 3. A thermoelectric couple comprising, mem Undef cOmPleSSiOD- (a) first and second semiconductor thermoelements, one

of which is doped with P-type impurities and one of 40 References Cited which is doped with N-type impurities, said rst UNITED STATES PATENTS thermoelement including a semiconductive mate- 2,858,350 10/1958 Fritts et al. 136-228 lilal ghich islnott sensitive to the environment 1n which 2,952,725 9/1960 Evans et aL 136 228 e errnoe'ec ric generator operates, which possess- 45 2,972,654 2/1961 Fritts et al. 13.6 228 es high tensile strength, and which is easily bondable 3,013,097 12/1961 Prius et aL 13,6 228 by mefaiiufglcal methode and 3,256,701 6/1966 Henderson 13e- 239 X (b) encasing means, including said rst thermoelement, 3,342,567 9/1967 Dingwau 13,; 239 X for isolating said second thermoelement from the en- 3,362,853 1/1968 Valdsaar 136-228 X vironment in which said thermoelectric couple operates, said encasing means further comprising, 5o FOREIGN PATENTS member which is a highly electrical and thermal conducting host semiconductor material doped with P-type impurities for receiving heat from a source of heat, for electrically coupling said first thermoelement to said second thermoelement, and

CARL D. QUARFORTH, Primary Examiner H. E. BEHREND, Assistant Examiner U.S. C1. X.R. 

